In light of the scarcity of natural resources and the increasing impact of CO2 on the environment, the current trend in automotive engineering is to use hybrid drive systems. The systems that are currently in use usually involve electric motor powered hybrids, which hybrids store the electric energy generated in braking modes and recover drive energy from the stored energy to provide assistance to the vehicle for the drive mode and, in particular, for acceleration processes. This strategy offers the possibility of decreasing the drive power of the internal combustion engine serving as the primary drive for comparable road performance. The result of such “downsizing” is not only a reduction in fuel consumption, but it also raises the possibility of assigning the vehicles concerned to a lower emissions class that satisfies a lower-cost road tax category.
These goals can also be achieved with a hydraulic hybrid system owing to the high energy density and the compact design of hydraulic systems. To make available additional drive torque even at low speeds and starting from zero speed for acceleration processes or to provide assistance to the braking action during braking modes, a hydraulic hybrid system stores the hydraulic energy in a hydraulic accumulator by a pump/motor unit. This hydraulic energy can then be used, as required, as the drive energy when the pump/motor unit is in motor mode. This hydrostatic drive system with recovery of the braking energy is disclosed in document DE 601 18 987 T2.